A good costume always starts with a solid idea. You may, as I do sometimes, peruse book or two specific to the type of costume you are trying to create. The internet can also provide inspiration in the form of image searches. I like to sketch out my costume, or sometimes costumes if my idea vary enough, after I have a good idea about what I want. Next, I will usually try to find a sewing pattern that is pretty close to what I have in mind and make alterations to make it just as I want. Such alterations are usually pretty simple, add some frills here and there and such; though even I have to ask for help from experts sometimes. And lastly, accessories and adding other little details will help to customize the costume and make it your own.